The invention relates to a process for producing symbols by laser irradiation on effect coatings or molding compounds comprising thermoplastic cholesteric liquid-crystal polymers (cLCPs) having optically variable properties.
It is known to inscribe surfaces of plastic by means of laser radiation, and in this context use is made, in addition to CO.sub.2 lasers, preferably of Nd:YAG lasers (wavelength 1064 nm) or a frequency-doubled Nd:YAG laser (532 nm). In the case of the laser markings customary at present, the marking mechanism is normally a thermal process (N. J. Bruton, Opt. Photonics News (1997), 8(5), 24-30). The absorbed laser light induces a rapid local temperature rise in the polymer material at the irradiated sites. The polymer material pyrolyzes and a marking becomes visible. In the case of polymers which are transparent to the light wavelength used it is necessary to use light-absorbing additives, such as kaolin.
Initially, only light-colored inscriptions on a dark background or dark inscriptions on a light-colored background were possible. Through the incorporation of colored pigments which are not destroyed by the laser irradiation, it is possible to generate colored laser markings (EP 0 641 821 A1).
The laser markings known to date are therefore distinguished by the fact that they possess, on a white or colored polymer, a marking in a different color. Neither the color of the plastic nor that of the inscription, however, possesses any viewing-angle dependency. Cholesteric main-chain polymers (cLCPs) are known and can be prepared in analogy to nematic main-chain polymers by using an additional chiral comonomer (U.S. Pat. No. 4,412,059; EP-A-0 196 785; EP-A-0 608 991; EP-A-0 391 368) or by reacting nematic main-chain polymers (LCPS) with additional chiral comonomers (EP-A-0 283 273). A feature of cholesteric main-chain polymers is their helical superstructure. As a result of this, first, the material no longer has the anisotropy of the mechanical properties which is typical of nematic liquid-crystal polymers. Depending on the chiral monomer content, the material exhibits pronounced color effects which are based on the selective reflection at the helical superstructure. The precise reflection color in this case depends on the viewing angle and above all on the pitch of the helix. For any given viewing angle--for example, a sample viewed vertically from above--the reflection color which appears is a color having a wavelength which corresponds to the pitch of the helical superstructure. This means that the wavelength of light reflected is shorter the smaller the pitch of the helix. The developing pitch of the helix depends essentially on the proportion of the chiral comonomer, on the nature of its incorporation into the polymer, on the degree of polymerization, and on the structure of the chiral comonomer.
Thin films of cholesteric liquid crystals exhibit pronounced color effects when applied to an absorbent substrate, especially a black substrate, since otherwise, owing to an inadequately pronounced hiding power of the cholesteric liquid crystals, the nonselective component of the light is reflected at the substrate, which attenuates the perceived color. It is known, furthermore, that instead of a black substrate it is also possible to use substrates colored otherwise. In addition, it is possible to influence the hiding power and perceived color of the cholesteric layer by the incorporation of colorants (DE-A-196 43 277).